1. Field of the Invention
The present invention relates to an optical disc recording apparatus that performs writing on an optical disc by use of laser. In particular, the present invention relates to an optical disc recording apparatus that performs writing on an optical disc for which an optimum write power is set, and to a method for determining the optimum write power in the optical disc recording apparatus.
2. Description of the Related Art
In a method of determining and controlling the optimum write power in optical disc recording (hereafter, optimum write power is abbreviated as Po, optimum power control is abbreviated as OPC), generally Po is calculated by arithmetically processing the write characteristics such as the fluctuation of reflectance at the time of writing, the asymmetry after writing, the amplitude, the depth of modulation and the jitter obtained by test writing on several points selected in the vicinity of the write power (hereafter, abbreviated as Pm) suitable for obtaining Po (hereafter, the series of processes is abbreviated as Po-operation).
The exact value of Pm, however, is not available until the OPC is performed. For this reason, test writing is performed with write powers in a range as broad as what can be estimated, Pm is selected on the basis of the write characteristics thus obtained and Po-operation is performed using the write characteristics of the points in the vicinity of Pm thus selected.
In OPC, the test writing is performed in consecutive recording tracks of optical disc while the disc is rotated. The test writing is performed with the write power being changed. In related art, however, the write power is changed simply from the lowest to the highest, or vice versa. As a result, there is a problem that the write power being dependent on the position in the optical disc.
FIG. 1 schematically shows an OPC method in related art. For a simple explanation, the example taken in this case includes a write sensitivity fluctuation in a sine wave pattern with a frequency θt. The number of powers with which the test writing is performed (hereafter, the number is abbreviated as N) is six in this example, but the number may vary for each actual case. P(i) is a series of test write power, E(i) is a series of write characteristics obtained by performing test writing with P(i). For an easy understanding of the explanation, the write power in the write power series P(i) is set to increase as the value i increases in this example, but the present invention is not limited to this.P(i) i:0 to N−1  (1)P(i)<P(i+1)  (2)
In general, in an OPC, test writing is performed in a specified, continuous position of a rotating disc with the write power being changed. FIG. 1 shows that a test writing is performed from a test-writing starting position indicated as θs with write powers P(0) to P(5). Subsequently, Po-operation is performed using write characteristics E(i) thus obtained. This example shows that {P(1)+P(2)}/2 is selected as Pm, and the write characteristics E(1) and E(2) in the vicinity of Pm are selected for Po-operation. With such write sensitivity fluctuation as illustrated in the figure, the Po-value would be a value corresponding to a state in which the write sensitivity obtained by a Po-operation is high because the write characteristics E(1) and E(2) used in the Po-operation are values measured in positions of high write sensitivities.
Every time test writing is performed, the phase relationship between the write sensitivity fluctuation and the test-writing starting position θs differs. FIG. 2 shows such an example. When OPC is performed under such a phase relationship, Po-value thus obtained is a value corresponding to a state in which the write sensitivity is low because the write characteristics E(1) and E(2) are values measured in positions with low write sensitivities.
Examples in FIG. 1 and FIG. 2 show a problem that the OPC in related art has. Specifically, the obtained Po-value changes as determined by the phase relationship between the write sensitivity fluctuation and test writing starting position, which results in an OPC with lower accuracy.
FIG. 23 shows an OPC method in related art. The circular arc arrows arranged in a substantially concentric manner on an optical disc 1 show how the test writing is performed on spiral recording tracks formed on an optical disc 1. The numbers put in a position closer to the center respectively indicate the positions on the optical disc 1. P(i) is a write power, with which test writing is performed in each section shown by the corresponding circular arch arrow. For a simple explanation, the number of write powers (N) in a write power series is set to be eight in this figure, but the number may vary for each actual case. Also for a simple explanation, the write power P(i) is set to increase as the value of i increases.
FIG. 24 shows the relationship, obtained by the OPC, among the position in disc, the write power, and the write characteristics. Here, E(i) is write characteristics at the time when the test writing is performed with the corresponding one of write powers P(i). As described above, in the OPC, the write power Pm that is suitable for obtaining Po is selected, then several points in the vicinity of the Po are selected, and then the Po-operation is performed. The example in FIG. 24 shows that P(3) is selected as the Pm, the write powers P(2), P(3) and P(4) in the vicinities of the selected Pm, and the write characteristics E(2), E(3) and E(4) are selected.
Incidentally, uneven write sensitivity occurs in an optical disc for various reasons. Such unevenness includes unevenness due to the characteristics of the recording film itself, unevenness due to the characteristics of a cover film and a reflective film, as well as unevenness due to contamination by fingerprints. Focusing on the influence on the OPC, apparent unevenness due to the misalignment of the relative position and the relative angle of the optical disc with the optical lens, caused by the bending of the optical disc and the attachment error is also included.
Most of such unevenness depends on the position in the optical disc. For this reason, in the test writing as performed in related art, namely the test writing with the write power being changed simply from the largest to the smallest, or vice versa, the regions where writing is performed with a write power in the vicinity of Pm becomes close to one another. Accordingly, a problem arises that some combinations of the write power and the position of uneven write sensitivity results in big changes in the number of measurement points influenced by the unevenness in write sensitivity.
FIG. 25 shows an example of distribution of uneven write sensitivity on an optical disc. FIG. 26 shows the relationship between the position in disc, the write power and the write characteristics, all of which are obtained by an OPC performed on an optical disc that has an uneven write sensitivity distribution shown in FIG. 25. FIG. 25 shows that uneven write sensitivity exists in an area 11. In such a case, all of the write characteristics E(2), E(3) and E(4) used in the Po-operation influenced by the uneven write sensitivity, and the accuracy of the Po calculated using these values lowers significantly.
FIG. 40 schematically shows an OPC method in related art. Here, P(i) indicates a write power, with which test writing is performed. The circular arc arrows arranged in a substantially concentric manner on the optical disc 1 show how the test writing is performed on recording tracks, while the numbers (0, 1, 2, 3, 4, 5 . . . ) put in a position closer to the center respectively indicate the positions on the optical disc 1. As shown in the figure, the test writing in OPC is performed in continuous regions of the rotating optical disc 1, with the write power being changed. For a simple explanation, as the following formula show, the write powers P(i) are set to monotonically increase as the value of i increases. The number of write power (N) in the write power series is set to be six in this case, but a variety of variables may be used in actual cases of OPC.P(i) i:0 to N−1where P(i)<P(i+1)
When the test writing is performed, the write characteristics fluctuate in accordance with the write sensitivity of the disc. FIG. 41 shows how the fluctuation occurs. The horizontal axis of each of the three graphs (1) (2) and (3) shows the position on the disc in FIG. 40. The vertical axes of the graphs (1), (2) and (3) from the top are the write power P, write sensitivity S, and write characteristics E, respectively. The third graph (3) from the top shows an example in which the write characteristics shifts from the values in theory indicated by the broken line to the values in practice indicated by the solid line as the uneven write sensitivity affects the write characteristics. Since Po is calculated using these write characteristics in OPC, a problem arises in related art in which uneven write sensitivity causing low accuracy of Po.
This uneven write sensitivity includes not only unevenness in characteristics of a recording film and a cover layer in the disc, but also apparent unevenness such as write-power loss due to contamination and misalignment of the optical axis.
Various methods have been proposed thus far to compensate for the influence of such write sensitivity fluctuation and of uneven write sensitivity. In these methods, however, compensating operation is preceded by measurement of write characteristics in areas in which test writing is to be performed (see, for example, Japanese Patent Application Laid-Open No. 2004-253016 and Japanese Patent Application Laid-Open No. 2002-319135), or by stabilization of the write characteristics on the area (see, for example, Japanese Patent Application Laid-Open No. 2000-251254). These methods have disadvantages in that since a series of operation including the measuring of characteristics and the writing is repeated, the processing of OPC as a whole tends to become complicated. Another problem is that a method including a preceding writing in an area where test writing is performed as a pre-processing is not suitable for an optical disc that is not rewritable such as CD-R (CD-Recordable).